As shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station to which the client terminal is communicating with is referred as the serving base station. In some wireless communication systems the serving base station is normally referred as the serving cell. The terms base station and a cell may be used interchangeably herein. In general, the cells that are in the vicinity of the serving cell are called neighbor cells. Similarly, in some wireless communication systems a neighbor base station is normally referred as a neighbor cell.
Client terminals used in wireless communication systems are required to search for the network, acquire the network information, camp on to the network and register for service. The aforementioned process is collectively called “network registration.”
The network registration process may normally take place in different scenarios that include but are not limited to powering on the client terminal, attempting to obtain service after a loss of network coverage (e.g., a dropped call due to a “dead spot” in the network), and when roaming from one network to another.
Since the client terminal is initially not synchronized with any of the base stations, it must first find the synchronization information such as the air interface timing and frequency. In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system, the network may use a number of different channel bandwidths. Also, the radio frame number (RFN) and the Physical Hybrid Automatic Repeat Request (HARQ) Indicator Channel (PHICH) configuration information is required for the client terminal to receive further details about the network. The above information is transmitted by each BS in the Physical Broadcast Channel (PBCH). The payload inside the PBCH is referred as Master Information Block (MIB). The MIB is used for further processing in the client terminal for network registration.
The 3GPP LTE wireless communication system air interface is organized into frames, subframes, and Orthogonal Frequency Division Multiplexing (OFDM) symbols as shown in FIG. 2, where the frame duration is 10 ms, the subframe duration is 1 ms and an OFDM symbol duration is in the range of 70 μs to 85 μs depending on the air interface configuration. The PBCH is transmitted over a duration of four OFDM symbols in subframe number zero and it is transmitted in every radio frame. The payload of the PBCH does not change over a period of four radio frames as shown in FIG. 3.
This allows the client terminal to perform combining of the PBCH over four radio frames as shown in FIG. 4. However, the PBCH contains the RFN in the payload (MIB) and the change of the RFN in payload occurs every four frames on a boundary where RFN modulo four is equal to zero as shown in FIG. 4. The RFN in the MIB contains only the upper eight most significant bits. The two least significant bits are zero for the frame where the change of the MIB content occurs. Since the client terminal is not aware of the RFN, the combining must be done over a period of seven frames while pursuing multiple parallel hypotheses as shown in FIG. 5. Each hypothesis starts at a new radio frame and corresponds to the two least significant bits of the RFN equal to zero. Only one of the four hypotheses can be correct and in the worst case it may be the last hypothesis that may be correct. As shown in FIG. 5, the hypothesis 4 is correct as the RFN is 104, which has two least significant bits equal to zero. Therefore, the worst case time required for one complete PBCH decode attempt for one cell is seven frames (7*10=70 ms).
The radio frame and subframe boundary are detected by the client terminal during the cell search procedure by first detecting the Primary Synchronization Signal (PSS) and then Secondary Synchronization Signal (SSS) as shown in FIG. 2. The PSS and SSS detection timing is relative to the internal timing of the client terminal and it is referred to herein as timing offset. The radio frame and subframe start timing is derived from the timing offsets of the detected PSS and SSS. The SSS detection requires the PSS time offset as an input from the PSS detection procedure. Therefore, the SSS detection may be scheduled only after successful PSS detection.
The timing of the receive window for decoding PBCH is based on the radio frame and subframe timing detected for a given cell based on SSS detection for that cell. Therefore, the PBCH detection may be scheduled only after successful SSS detection for that cell. In the remainder of the present disclosure whenever SSS detection is scheduled it is implicit that it is preceded by a successful PSS detection. Similarly, whenever PBCH detection is scheduled it is implicit that it is preceded by a successful SSS detection. Since the radio frame duration is 10 ms, the largest SSS timing offset can at most be 10 ms relative to the internal timing of the client terminal.
In cellular communication systems that employ frequency reuse, different cells may use the same Radio Frequency (RF) channel. When the client terminal performs SSS detection on a given RF channel, it may detect the SSS signals from multiple cells that are using the same RF channel. In addition to the radio frame and subframe timing of a cell, the SSS detection provides Physical Cell Identity (PCI), and may provide metrics such as signal strength and signal quality such as the Signal to Interference plus Noise Ratio (SINR) for each of the detected cell identified by its PCI. The various SSS detection metrics of a cell are collectively referred herein as a SSS detection report.
The next step after SSS detection is the PBCH decoding. The detection reports of cells for which the SSS detection was successful may be arranged in descending order according to different metrics such as estimated SINR, estimated SSS signal power, etc. Typically the cell with the highest ranked (e.g., “strongest”) SSS detection report may be selected for scheduling the PBCH decoding for that cell. However, the cell with the highest ranked or strongest SSS detection report may not necessarily lead to successful PBCH decode or may not be a suitable cell for network registration. In this case the client terminal may select another cell with next strongest SSS detection report and schedule the PBCH detection for that cell. This process continues until a successful PBCH decode, followed by a successful System Information (SI) decode, is performed for a cell that is suitable based on the decoded SI. This process of decoding the PBCH for different cells is sequential and may cause the client terminal to take a longer time to register to the network and be available for service. This can be disadvantageous as the longer it takes to register the more power is consumed in the client device. It may also degrade the user experience. The decoding of PBCH for multiple cells is required in many scenarios including cell reselection and handovers. For example, in case of femtocells deployed with restricted access using Closed Subscriber Group (CSG) feature, the need for decoding PBCH in a timely manner for multiple cells is critical. The femtocells are often referred as Home evolved Node-B (HeNB) in the 3GPP specification.